Re: [PATCH v3 5/6] kasan: Unset panic_on_warn before calling panic()

Kees Cook
2020-01-17T21:20:04Z

On Fri, Jan 17, 2020 at 10:54:36AM +0100, Dmitry Vyukov wrote:
> On Fri, Jan 17, 2020 at 12:49 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Jan 16, 2020 at 06:23:01AM +0100, Dmitry Vyukov wrote:
> > > On Thu, Jan 16, 2020 at 2:24 AM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > As done in the full WARN() handler, panic_on_warn needs to be cleared
> > > > before calling panic() to avoid recursive panics.
> > > >
> > > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > > ---
> > > > mm/kasan/report.c | 10 +++++++++-
> > > > 1 file changed, 9 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > > > index 621782100eaa..844554e78893 100644
> > > > --- a/mm/kasan/report.c
> > > > +++ b/mm/kasan/report.c
> > > > @@ -92,8 +92,16 @@ static void end_report(unsigned long *flags)
> > > > pr_err("==================================================================\n");
> > > > add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> > > > spin_unlock_irqrestore(&report_lock, *flags);
> > > > - if (panic_on_warn)
> > > > + if (panic_on_warn) {
> > > > + /*
> > > > + * This thread may hit another WARN() in the panic path.
> > > > + * Resetting this prevents additional WARN() from panicking the
> > > > + * system on this thread. Other threads are blocked by the
> > > > + * panic_mutex in panic().
> > >
> > > I don't understand part about other threads.
> > > Other threads are not necessary inside of panic(). And in fact since
> > > we reset panic_on_warn, they will not get there even if they should.
> > > If I am reading this correctly, once one thread prints a warning and
> > > is going to panic, other threads may now print infinite amounts of
> > > warning and proceed past them freely. Why is this the behavior we
> > > want?
> >
> > AIUI, the issue is the current thread hitting another WARN and blocking
> > on trying to call panic again. WARNs encountered during the execution of
> > panic() need to not attempt to call panic() again.
>
> Yes, but the variable is global and affects other threads and the
> comment talks about other threads, and that's the part I am confused
> about (for both comment wording and the actual behavior). For the
> "same thread hitting another warning" case we need a per-task flag or
> something.

This is duplicating the common panic-on-warn logic (see the generic bug
code), so I'd like to just have the same behavior between the three
implementations of panic-on-warn (generic bug, kasan, ubsan), and then
work to merge them into a common handler, and then perhaps fix the
details of the behavior. I think it's more correct to allow the panicing
thread to complete than to care about what the other threads are doing.
Right now, a WARN within the panic code will either a) hang the machine,
or b) not panic, allowing the rest of the threads to continue, maybe
then hitting other WARNs and hanging. The generic bug code does not
suffer from this.

-Kees

>
> > -Kees
> >
> > >
> > > > + */
> > > > + panic_on_warn = 0;
> > > > panic("panic_on_warn set ...\n");
> > > > + }
> > > > kasan_enable_current();
> > > > }
> >
> > --
> > Kees Cook

--
Kees Cook